Although AIDS-defining illnesses have decreased, the prevalence of HIV-associated non-AIDS conditions such as HIV-Associated Neurocognitive Disorders (HAND) remains high and is estimated to be over 50%, particularly in aging individuals with long-standing HIV infection. However, the pathophysiology of HAND in aging HIV+ adults remains unresolved. Current evidence and our preliminary data suggest that interactions of altered gut microbiome (dysbiosis), gut-derived microbial translocation, and systemic inflammation contribute to neurodegenerative processes. It is becoming increasingly evident that in both HIV-1 infection and aging, alterations in gut microbiome (dysbiosis) and ensuing increase in intestinal permeability and microbial translocation (MT) are major pathogenic drivers of local and systemic inflammation. Importantly, aging- associated microbiota changes are shown to be connected to immunosenescence and inflammaging. Preclinical/clinical studies using bacterial 16S ribosomal RNA (rRNA) gene sequencing, indicate that microbial dysbiosis associated with HIV-1 infection or aging has several common pathogenic features. However, these studies were largely hypothesis-generating with limited sample sizes, and were not adequately powered to address microbiome endpoints after correction for multiple testing, and did not reveal the functional potential of the microbiota (pathogenic or beneficial), or yield bacterial resolution to species or strain level. The current proposal will address these limitations by using an adequately powered longitudinal study and will conduct 16S rRNA gene and Whole Genome Shotgun (WGS) metagenomic sequencing that will determine bacterial composition and diversity, provide identification at the species and strain level, and enable the functional characterization of the bacterial genes. Our overarching hypothesis is that the interactive effects of aging and HIV-1 infection at the level of gut dysbiosis and permeability, and ensuing local and systemic inflammation play a major pathogenic role in driving HIV infection and aging-associated neuroinflammation and cognitive dysfunction. To test these hypotheses, we will leverage and utilize HIV+ and healthy aging populations from ongoing NIH-sponsored longitudinal studies at the Universities of Louisville (UofL) and Florida (UF) with the following specific aims: Aim 1: To assess longitudinal qualitative and quantitative changes in the gut microbiome (dysbiosis) in older persons living with HIV-1 infection. Aim 2: To determine the impact of HIV-1 infection and age associated gut dysbiosis on (A) intestinal permeability and microbial translocation (MT), and resultant peripheral endotoxemia, and inflammation; and (B) multimodal MRI/MRS measures of neuroinflammation and cerebral metabolic disturbance. Aim 3: To investigate the impact of gut dysbiosis and peripheral and neuroinflammation, and cerebral metabolic disturbance on cognitive dysfunction and functional brain abnormalities (FMRI) relative to age and HIV status.